Not sure what to do, but I would start by not using a retractible leash. This will give you a lot more control, and use the command "close" a lot with a reward. Hopefully, the dog will figure out it's better to stay close to you and get a treat than to chase cars.
My neighbor has the same problem, and when he locks that leash the dog is often at full speed and it looks like his head is going to pop off.
